Loops In Solar Active Regions Research Articles

Damped kink motions in a system of two solar coronal tubes with elliptic cross sections

Aims. This study is motivated by observations of coordinated transverse displacements in neighboring solar active region loops, addressing specifically how the behavior of kink motions in straight two-tube equilibria is impacted by tube interactions and tube cross-sectional shapes. Methods. We worked with linear, ideal, pressureless magnetohydrodynamics. Axially standing kink motions were examined as an initial value problem for transversely structured equilibria involving two identical, field-aligned, density-enhanced tubes with elliptic cross sections (elliptic tubes). Continuously nonuniform layers were implemented around both tube boundaries. We numerically followed the system response to external velocity drivers, largely focusing on the quasi-mode stage of internal flows to derive the pertinent periods and damping times. Results. The periods and damping times that we derive for two-circular-tube setups justify the available modal results found with the T-matrix approach. Regardless of cross-sectional shapes, our nonuniform layers feature the development of small-scale shears and energy accumulation around Alfvén resonances, indicative of resonant absorption and phase mixing. As with two-circular-tube systems, our configurational symmetries still make it possible to classify lower-order kink motions by the polarization and symmetric properties of the internal flows; hence, such motions are labeled as S​x and Ax. However, the periods and damping times for two-elliptic-tube setups further depend on cross-sectional aspect ratios, with Ax motions occasionally damped less rapidly than S​x motions. We find uncertainties up to ∼20% (∼50%) for the axial Alfvén time (the inhomogeneity lengthscale) if the periods (damping times) computed for two-elliptic-tube setups are seismologically inverted with canonical theories for isolated circular tubes. Conclusions. The effects of loop interactions and cross-sectional shapes need to be considered when the periods, and in particular the damping times, are seismologically exploited for coordinated transverse displacements in adjacent coronal loops.

Generalized Coronal Loop Scaling Laws and Their Implication for Turbulence in Solar Active Region Loops

Recent coronal loop modeling has emphasized the importance of combining both Coulomb collisions and turbulent scattering to characterize field-aligned thermal conduction, which invokes a hybrid loop model. In this work, we generalize the hybrid model by incorporating a nonuniform heating and cross section that are both formulated by a power-law function of temperature. Based on the hybrid model solutions, we construct scaling laws that relate loop-top temperature (T a ) and heating rate (H a ) to other loop parameters. It is found that the loop-top properties for turbulent loops are additionally power-law functions of the turbulent mean free path (λ T ), with the functional forms varying from situation to situation, depending on the specification of the heating and/or areal parameters. More importantly, both a sufficiently footpoint-concentrated heating and a cross-sectional expansion with height can effectively weaken (strengthen) the negative (positive) power-law dependence of T a (H a ) on λ T . The reason lies in a notable reduction of heat flux by footpoint heating and/or cross-sectional expansion in the turbulence-dominated coronal part, where turbulent scattering introduces a much weaker dependence of the conduction coefficient on temperature. In this region, therefore, the reduction of the heat flux predominately relies on a backward flattening of the temperature gradient. Through numerical modeling that incorporates more realistic conditions, this scenario is further consolidated. Our results have important implications for solar active region (AR) loops. With the factors of nonuniform heating and cross section taken into account, AR loops can bear relatively stronger turbulence while still keeping a physically reasonable temperature for nonflaring loops.

Standing Sausage Perturbations in Solar Coronal Slabs with Continuous Transverse Density Profiles: Cutoff Wavenumbers, Evanescent Eigenmodes, and Oscillatory Continuum

The lack of observed sausage perturbations in solar active region loops is customarily attributed to the relevance of cutoff axial wavenumbers and the consequent absence of trapped modes (called “evanescent eigenmodes” here). However, some recent eigenvalue problem studies suggest that cutoff wavenumbers may disappear for those equilibria where the external density varies sufficiently slowly, thereby casting doubt on the rarity of candidate sausage perturbations. We examine the responses of straight, transversely structured coronal slabs to small-amplitude sausage-type perturbations that excite axial fundamentals, by solving the pertinent initial value problem with eigensolutions for a closed domain. The density variation in the slab exterior is dictated by some steepness parameter μ, and cutoff wavenumbers are theoretically expected to be present (absent) when μ ≥ 2 (μ < 2). However, our numerical results show no qualitative difference in the system evolution when μ varies, despite the differences in the modal behavior. Only oscillatory eigenmodes are permitted when μ ≥ 2. Our discrete eigenspectrum becomes increasingly closely spaced when the domain broadens, and an oscillatory continuum results for a truly open system. Oscillatory eigenmodes remain allowed and dominate the system evolution when μ < 2. We show that the irrelevance of cutoff wavenumbers does not mean that all fast waves are evanescent. Rather, it means that an increasing number of evanescent eigenmodes emerge when the domain size increases. We conclude that sausage perturbations remain difficult to detect, even for the waveguide formulated here.

Temperature and Differential Emission Measure Profiles in Turbulent Solar Active Region Loops

We examine the temperature structure of static coronal active region loops in regimes where thermal conductive transport is driven by Coulomb collisions, by turbulent scattering, or by a combination of the two. (In the last case collisional scattering dominates the heat transport at lower levels in the loop where temperatures are low and densities are high, while turbulent scattering dominates the heat transport at higher temperatures/lower densities.) Temperature profiles and their corresponding differential emission measure distributions are calculated and compared to observations, and earlier scaling laws relating the loop apex temperature and volumetric heating rate to the loop length and pressure are revisited. Results reveal very substantial changes, compared to the wholly collision-dominated case, to both the loop scaling laws and the temperature/density profiles along the loop. They also show that the well-known excess of differential emission measure at relatively low temperatures in the loop may be a consequence of the flatter temperature gradients (and so increased amount of material within a specified temperature range) that results from the predominance of turbulent scattering in the upper regions of the loop.

Measurements of Coronal Magnetic Field Strengths in Solar Active Region Loops

Abstract The characteristic electron densities, temperatures, and thermal distributions of 1 MK active region loops are now fairly well established, but their coronal magnetic field strengths remain undetermined. Here we present measurements from a sample of coronal loops observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode. We use a recently developed diagnostic technique that involves atomic radiation modeling of the contribution of a magnetically induced transition to the Fe x 257.262 Å spectral line intensity. We find coronal magnetic field strengths in the range of 60–150 G. We discuss some aspects of these new results in the context of previous measurements using different spectropolarimetric techniques, and their influence on the derived Alfvén speeds and plasma β in coronal loops.

Synthetic Extreme-ultraviolet Emissions Modulated by Leaky Fast Sausage Modes in Solar Active Region Loops

We study the extreme-ultraviolet (EUV) emissions modulated by leaky fast sausage modes (FSMs) in solar active region loops and examine their observational signatures via spectrometers like EIS. After computing fluid variables of leaky FSMs with MHD simulations, we forward-model the intensity and spectral properties of the Fe X 185~\AA~and Fe XII 195~\AA~lines by incorporating non-equilibrium ionization (NEI) in the computations of the relevant ionic fractions. The damping times derived from the intensity variations are then compared with the wave values, namely the damping times directly found from our MHD simulations. Our results show that in the equilibrium ionization cases, the density variations and the intensity variations can be either in phase or in anti-phase, depending on the loop temperature. NEI considerably impacts the intensity variations but has only marginal effects on the derived Doppler velocity or Doppler width. We find that the damping time derived from the intensity can largely reflect the wave damping time if the loop temperature is not drastically different from the nominal formation temperature of the corresponding emission line. These results are helpful for understanding the modulations to the EUV emissions by leaky FSMs and hence helpful for identifying FSMs in solar active region loops.

Extracting the Temperature of a Coronal Loop in the Solar Active Region 11092

Extracting the temperature of coronal loops is effective in the analysis of solar active region's loops and helps in better understanding of coronal events. To this end, various methods have already been developed like the method developed by Aschwanden et al. 2015 \cite{AschwandenB2015} which is based on Gaussian fit for Differential Emission Measure (DEM). Here, we use the intensity ratios of the images in three different wavelengths and the temperature response functions of AIA to extract the temperature of the loop. In this paper, we use EUV images of the solar active region 11092 taken by AIA instrument of the SDO satellite at 171, 193, and 211 $A^{0}$ wavelengths, at 1th of August, 2010. We select a loop in a subregion of 11092 and extract its temperature by the help of intensity profiles in different wavelengths and thermal response functions of different filters. In this subregion cooling of the loop happens and in the selected loop, highest relative intensity of the wavelengths of 171 $A^{0}$ to 193 $A^{0}$ was obtained to be $0.76$ and this number was estimated to correspond to the temperature of 1.3 million degree of Kelvin, which is the maximum temperature point of this loop's internal area. The highest values of the intensity ratios at the wavelengths of 211$A^{0}$ to 193$A^{0}$, and 211$A^{0}$ to 171$A^{0}$ are $0.22$ and $0.25$, which correspond to temperature values around 10M and 1.4M Kelvin in sequence, related to the temperature of hotter and more superficial points of the loop respectively. These values are very sensitive to time and differ in time series of this event of the loop intensity variation.

Properties of the Diffuse Emission around Warm Loops in Solar Active Regions

Abstract Coronal loops in active regions are the subjects of intensive investigation, but the important diffuse unresolved emission in which they are embedded has received relatively little attention. Here I measure the densities and emission measure (EM) distributions of a sample of background–foreground regions surrounding warm (2 MK) coronal loops, and introduce two new aspects to the analysis. First, I infer the EM distributions only from temperatures that contribute to the same background emission. Second, I measure the background emission co-spatially with the loops so that the results are truly representative of the immediate loop environment. The second aspect also allows me to take advantage of the presence of embedded loops to infer information about the (unresolvable) magnetic field in the background. I find that about half of the regions in my sample have narrow but not quite isothermal EM distributions with a peak temperature of 1.4–2 MK. The other half have broad EM distributions (Gaussian width &gt;3 × 105 K), and the width of the EM appears to be correlated with peak temperature. Densities in the diffuse background are log (n/cm−3) = 8.5–9.0. Significantly, these densities and temperatures imply that the co-spatial background is broadly compatible with static equilibrium theory (RTV scaling laws) provided that the unresolved field length is comparable to the embedded loop length. For this agreement to break down, the field length in most cases would have to be substantially longer than the loop length, a factor of 2–3 on average, which for the sample in this work approaches the dimensions of only the largest active regions.

The Heating of Coronal Loops in Solar Active Regions

The active region corona is believed to be heated by magnetic disturbances that propagate into the corona from the convection zone below. A reduced magnetohydrodynamic (MHD) model of Alfvén waves in a coronal loop is presented. The waves are launched in the photosphere from a collection of kilogauss flux tubes, and they reflect at various positions along the loop, leading to counter-propagating waves and turbulence. It is found that turbulent Alfvén waves can produce only enough heat to maintain a peak temperature of about 2.5 MK, less than the temperatures typically observed in active regions (∼ 4 MK). We consider an alternative model in which the flux tubes are subject to slow random footpoint motions, but we find that such braiding motions produce less heating than the waves inside the flux tubes. Therefore, models of coronal heating based on small-scale random footpoint motions cannot readily explain the observed high temperatures loops; more energetic “nanoflare” heating events are required. We suggest that such strong heating events may be produced by disturbances associated with flux emergence and large-scale non-potential fields.

DIAGNOSTICS OF CORONAL HEATING IN ACTIVE-REGION LOOPS

ABSTRACT Understanding coronal heating remains a central problem in solar physics. Many mechanisms have been proposed to explain how energy is transferred to and deposited in the corona. We summarize past observational studies that attempted to identify the heating mechanism and point out the difficulties in reproducing the observations of the solar corona from the heating models. The aim of this paper is to study whether the observed extreme ultraviolet (EUV) emission in individual coronal loops in solar active regions can provide constraints on the volumetric heating function, and to develop a diagnostic for the heating function for a subset of loops that are found close to static thermal equilibrium. We reconstruct the coronal magnetic field from Solar Dynamics Observatory/HMI data using a nonlinear force-free magnetic field model. We model selected loops using a one-dimensional stationary model, with a heating rate dependent locally on the magnetic field strength along the loop, and we calculate the emission from these loops in various EUV wavelengths for different heating rates. We present a method to measure a power index β defining the dependence of the volumetric heating rate E H on the magnetic field, , and controlling also the shape of the heating function: concentrated near the loop top, uniform and concentrated near the footpoints. The diagnostic is based on the dependence of the electron density on the index β. This method is free from the assumptions of the loop filling factor but requires spectroscopic measurements of the density-sensitive lines. The range of applicability for loops of different length and heating distributions is discussed, and the steps to solving the coronal heating problem are outlined.

Heating of coronal active regions

AbstractRecent observations of coronal loops in solar active regions show that their heating must be a truly dynamic process. Even though it seems clear that the energy source is the magnetic field that confines the coronal plasma, the details of how it dissipates are still a matter of debate. In this presentation we review the theoretical models of coronal heating, which have been traditionally clasified as DC or AC depending on the electrodynamic response of the loops to the photospheric driving motions.Also, we show results from numerical simulations of the internal dynamics of coronal loops within the framework of the reduced MHD approximation. These simulations indicate that the application of a stationary velocity field at the photospheric boundary leads to a turbulent stationary regime after several photospheric turnover times. Once this turbulent regime is set, both DC and AC stresses dissipate at faster rates as a result of a direct energy cascade.

A reinterpretation of the energy balance in active region loops following new results from Hinode EIS

Context. New observations made by the Hinode EUV Imaging Spectrometer have revealed persistent redshifts in solar active region loops in the temperature range 10 5.6 ≤ T ≤ 10 6.4 K. The presence of redshifts, interpreted as bulk downflows, indicates that the loops are undergoing radiative cooling rather than continuous heating. This has significant consequences for current ideas regarding the physics of the ubiquitous 1 MK loops observed by instruments such as TRACE and SoHO-EIT. Aims. The aim of this work is to assess the validity of the radiative cooling interpretation of the physics governing these loops. Methods. This interpretation requires an enthalpy flux to balance the radiative energy loss in order to avoid catastrophic cooling. An analytical model is developed and used to find the downflows needed to drive an enthalpy flux sufficient to balance the known radiative loss rates, which are then compared with the observationally measured redshifts. Results. The model results are found to be in good agreement with the observed redshifts. Conclusions. Based upon the current analysis it appears that the majority of active region loops observed in the temperature range 10 5.6 ≤ T ≤ 10 6.4 K are in the radiative cooling phase of their lifecycle.

The Formation of Coronal Loops by Thermal Instability in Three Dimensions

Plasma loops in solar active regions have been observed in EUV and soft X-rays for decades. Their formation mechanism and properties, however, are still not fully understood. Predictions by early models, based on 1D hydrostatic equilibria with uniform plasma heating, are not consistent with high-resolution measurements. In this Letter, we demonstrate, via 3D simulations, that a class of heating models can lead to the dynamic formation of plasma loops provided the plasma is heated sufficiently to match SXT soft X-ray measurements. We show that individual flux tubes in a 3D magnetic structure tend to stand out against their neighbors. The loops have large aspect ratios and nearly uniform cross sections in the corona, similar to those observed by EIT and TRACE. The coronal EUV emission from these thermally unstable solutions is roughly consistent with EIT measurements. The solution oscillates in time through a large-amplitude, nonlinear cycle, leading to repeated brightening and fading of the loops.

The Sun as a MHD generator: application of a new heating mechanism for the coronal loops and closed magnetic structures

We investigate a possibility of heating of the loops and other closed magnetic structures in active regions of the solar corona by the flow of solar wind (plus other flows that may be present) across the magnetic field lines (that are perpendicular to the flow), in a similar manner as a conventional MHD generator works. A simple model is formulated which allows to calculate the typical currents generated by this mechanism. This enables to make a judgement whether the loops in solar active regions can be heated by the proposed mechanism through the associated Ohmic dissipation. It is shown that even if the heating region width is of the order of a few proton Larmor radii (this effectively means that a loop is represented by {\it nano-threads}, by analogy with the nano-flare coronal heating mechanism, with width of a few proton Larmor radii) and the plasma flow speed is 50 km s$^{-1}$ then only about 1% of the heating requirement of coronal loops could be met under this conjecture. Other implications of the model, such as a predicted spatial heat distribution along the loop as well as application of the model to the other places e.g. chromosphere and corotating interaction region structures in the solar wind are discussed.

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.

The Magnetic Corona: Magnetic Reconnection in Solar Flares

The ability of the Transition Region and Coronal Explorer (TRACE) to image the Sun at high spatial resolution and high cadence over a very broad range of temperatures makes it a unique instrument for observing solar flare plasma. TRACE observations have confirmed the reconnection model for solar flares, at least qualitatively. TRACE flare observations show impulsive footpoint brightenings that are followed by the formation of high-temperature loops in the corona. These loops then cool to lower temperatures, forming post-flare loop arcades. Comparisons between TRACE and lower spatial resolution Yohkoh Soft X-Ray Telescope (SXT) observations have revealed that solar flares are composed of a multitude of fine coronal loops. Detailed hydrodynamic modeling of flare light curves shows that this fine scale structuring is crucial to understanding the evolution of the observed emission. Models based on single, isothermal loops are not consistent with the TRACE observations. Models based on the sequential heating of small-scale loops, in contrast, are able to reproduce many of the salient features of the observed light curves. We will discuss the implication of these results for more energetic stellar flares as well as smaller-scale events that may be responsible for the heating of solar active region loops.

On the process of resistive heating instability and the formation of coronal loop structures

view Abstract Citations (8) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Process of Resistive Heating Instability and the Formation of Coronal Loop Structures Lou, Yu-Qing Abstract A cylindrical electric current sheath embedded in a coaxial magnetic flux rope is in an inductive, thermal, and mechanical equilibrium. The transport processes involved are assumed to be anomalous. Such a system is subject to a class of macroscopic resistive heating instabilities in addition to the known MHD mechanical, thermal, and tearing instabilities. The onset of the resistive heating instability within the sheath gives rise to various large-scale helical patterns of thermal perturbation winding along the background magnetic flux rope. These helical thermal structures roughly map out the general direction of the magnetic field lines within the sheath. In an idealized scenario, a solar coronal magnetic flux rope may consist of many coaxial current sheaths of different radii. The coupled development of the MHD resistive heating, mechanical, thermal, and tearing instabilities within such an ensemble of current sheaths may result in the appearance of compact X-ray loop patterns on virtually all spatial scales with various collective morphologies. We propose the resistive heating instability as an important elemental mechanism leading to the formation of compact X-ray loops in solar active regions. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171688 Bibcode: 1992ApJ...395..682L Keywords: Gamma Ray Astronomy; Magnetohydrodynamic Stability; Plasma Sheaths; Solar Corona; Solar X-Rays; Electric Current; Perturbation Theory; Solar Activity; Solar Magnetic Field; Solar Physics; MAGNETOHYDRODYNAMICS: MHD; SUN: CORONA; SUN: X-RAYS; GAMMA RAYS full text sources ADS | Related Materials (1) Erratum: 1993ApJ...404..418L

VLA Observations of the Coronal Plasma

VLA observations at 20-cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft X-ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio or X-ray wavelengths while remaining invisible in the other spectral domain, and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90-cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying Hα filament seen in absorption. The 20-cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources.

Coronal Loops in Solar Active Regions

AbstractWe discuss the appearance of solar active regions as seen in EUV line radiation in terms of a system of simple static coronal loop models. The loop models are based on those of Rosner et al. (1978) and may be understood in terms of the scaling laws introduced by these authors. We present here some generalized scaling laws which demonstrate that the global properties of loops in fixed length are insensitive to everything but the overall heating rate or the basal pressure. The model and scaling laws appear to describe hot loops (T&lt; 106K) well. Empirically determined temperatures and densities in cooler loops are not in agreement with coronal loop models.We have computed a simple model of an active region using a dipole magnetic structure. The individual loop structures were calculated on the basis of the two different assumptions of constant basal pressure and of constant heating rate. This paper presents a preliminary discussion of the results and a comparison with observation.